Some engines, known as variable displacement engines (VDE), may be configured to operate with a variable number of active and deactivated cylinders to increase fuel economy. Therein, a portion of the engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including engine temperature. An engine control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling (also referred to as a deceleration fuel shut-off event, or DFSO), and/or through the control of the ignition system to selectively control (e.g., withhold) spark to deactivatable cylinders. In some examples, an engine controller may continuously rotate the identity of cylinders that receive air and fuel, and those that are skipped, as well as vary a number of cylinder events over which a specific deactivation pattern is applied. By skipping air and fuel delivery to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine. By varying the identity and number of cylinders skipped, a large range of engine displacement options may be possible.
Still further improvements in fuel economy can be achieved by selectively deactivating a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and/or exhaust valves. One example of a deceleration cylinder cut-off operation (DCCO) is shown by Carlson et al. in U.S. Pat. No. 9,790,867. Therein, during conditions when engine torque is not required, each cylinder is deactivated in the working cycle following the DCCO entry decision by deactivating fuel and disabling air from being pumped through the cylinder valves.
However, the inventors herein have recognized potential issues with such systems. As one example, the DCCO operation may directly interfere with on-board diagnostic (OBD) routines that need to be completed to ensure emissions compliance. For example, exhaust catalysts and associated oxygen sensors may be diagnosed during a DFSO event by leveraging the air absent of fuel being pumped through the cylinders. Therein, a lean response to the DFSO is used to indicate that the catalyst and oxygen sensors are functioning as expected. Typically, OBD routines must be attempted a number of times, and completed another number of times, within a drive cycle for emissions to be considered compliant. Deactivating of cylinder valves results in no air being pumped through the cylinders to the exhaust system, making it difficult for the diagnostics to be performed.
In one example, the issues described above may be addressed by a method for operating an engine with selectively deactivatable cylinders, comprising: responsive to decreased torque demand, deactivating fuel to a cylinder, while maintaining valve operation; and further deactivating the valve operation of the cylinder responsive to completion of an exhaust catalyst and/or associated exhaust oxygen sensor diagnostic. In this way, the benefits of both a DFSO and a DCCO can be leveraged without compromising OBD completion and emissions compliance.
As one example, responsive to cylinder deactivation conditions being met, one or more engine cylinders may be selectively deactivated by disabling fuel (DFSO) and spark delivery while continuing to pump air through cylinder intake and exhaust valves. Once the DFSO is initiated, OBD routines may be conducted to diagnose one or more exhaust catalysts and exhaust oxygen sensors. For example, it may be determined if a lean response is observed at the exhaust catalysts and exhaust oxygen sensors. In some examples, prior to disabling, the cylinder fueling may be adjusted to enrich the downstream oxygen sensor and catalyst. Then after disabling the fueling, an engine controller may monitor the exhaust components for a rich-to-lean transition. Upon completion of the monitoring, such as upon confirming a lean response at a downstream HEGO sensor and exhaust catalyst, cylinder valves may be deactivated. In other words, the engine may be transitioned from the DFSO state to the DCCO state based on the exhaust response.
In this way, deactivation of cylinder fueling and cylinder valve operation can be better coordinated with the completion of OBD routines. The technical effect of diagnosing the performance of an exhaust catalyst and oxygen sensor after disabling cylinder fueling and before disabling cylinder valve operation is that the fuel economy benefits of both a DFSO operation and a DCCO operation can be achieved without compromising on emissions compliance. By delaying the DCCO operation until a rich-to-lean transition is observed at an exhaust oxygen sensor, catalyst and sensor monitoring may be attempted and completed during a drive cycle. Overall, engine fuel economy and exhaust emissions are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.